60 YEARS OF POMC: Adrenal and extra-adrenal functions of ACTH

1. View larger version:

   • In this window
   • In a new window

   • Download as PowerPoint Slide

   Figure 1

   Steroidogenesis in the three zones of the adrenal cortex. (A) Hematoxylin- and eosin-stained section of an adult rat adrenal gland. Scale bar, 100 μm. (B) Free cholesterol is recruited in three enzymatic pathways, leading to aldosterone in zona glomerulosa; corticosterone or cortisol in zona fasciculata and zona reticularis; and dehydroepiandrosterone (DHEA), DHEAS, and androstenedione in zona reticularis. Cholesterol is cleaved in the inner mitochondrial membrane by P450 cholesterol side-chain cleavage enzyme (P450scc/CYP11A1) into pregnenolone. Further steps involve the enzymes indicated in the figure. The steps indicated in red take place in the mitochondria and the steps indicated in blue take place in the endoplasmic reticulum. Data from Arlt & Stewart PM (2005).
2. View larger version:

   • In this window
   • In a new window

   • Download as PowerPoint Slide

   Figure 2

   Involvement of the extracellular matrix (ECM) and the cytoskeleton in ACTH-stimulated rat adrenal glomerulosa cells. (A) Immunofluorescence labeling of actin filaments and paxillin of rat glomerulosa cells, incubated without (Control) or with 10 nM ACTH for 5 min. Cells were processed for immunofluorescence labeling, using phalloidin coupled to Alexa-Fluor 594 nm for visualization of F-actin (red) and with anti-paxillin antibody coupled to Alexa-Fluor 488 nm for visualization of paxillin (green). Merged images are illustrated. Scale bars, 13 m. (B) Illustration of signaling pathways linked to ECM and cytoskeleton. In control conditions, binding of fibronectin or collagen to their integrins promotes strong cell adhesion, evidenced by the flat polygonal morphology, the thin stress fibers across the entire cell and by the presence of focal adhesion points revealed by paxillin labeling, as illustrated by the green fluorescent dots in the left part of panel A. ACTH induces a rapid but transient formation of a dense F-actin ring at the cell membrane, with disruption of the stress fiber network, as illustrated in the right part of panel A. These changes are accompanied by a dephosphorylation of paxillin at the plasma membrane and by the activation of the actin-associated kinases, such as the phosphotyrosine phosphatase, SHP2, which increase cell functionality.
3. View larger version:

   • In this window
   • In a new window

   • Download as PowerPoint Slide

   Figure 3

   Overview of the main signaling modules implicated in the effect of ACTH on adrenocortical cells. Regulation of ACTH action on adrenocortical cells may occur at different levels that can be divided into modules: Module 1, ACTH binding to its receptor, MC2R; Module 2, production of second messengers; Module 3, modulation of membrane channels; Module 4, implication of the extracellular matrix and cytoskeleton; Module 5, activation of various kinases and phosphatases; and finally Module 6, proteins and enzymes engaged in steroidogenesis or trophic action. Each of these modules could be considered as independent signaling cascades that interact through some of their elements, as illustrated in Fig. 4.
4. View larger version:

   • In this window
   • In a new window

   • Download as PowerPoint Slide

   Figure 4

   Illustrations of the main signaling cascades stimulated by ACTH, from binding to its receptor to cellular function in adrenocortical cells. (A) ACTH binds to MC2R and through interaction with MRAPs (Module 1) and initiates signaling, by activating Gs and various isoforms of ACs that increase cAMP. MC2R is also linked to G_i protein; activation of αi decreases the level of cAMP, whereas the release of βγ-subunits stimulates other effectors such as Mitogen-activated protein kinases (MAPK) cascade or cationic Cl⁻ channels (Module 2). Binding of cAMP to the regulatory subunits of protein kinase A results in the phosphorylation of several proteins, including steroidogenic acute regulatory protein (StAR) and the hormone-sensitive lipase. Protein kinase A (PKA) also regulates the level of expression of the receptors implicated in the uptake of cholesterol and genes encoding the steroidogenic enzymes (Module 5). The final output of this cascade is steroidogenesis, which is initiated in the mitochondria. cAMP also has a number of PKA-independent effects, including involvement of the exchange protein directly activated by cAMP (Epac1/2). cAMP also regulates its own intracellular level through activation of phosphodiesterases, in particular, PDE2 and PDE8 (Module 5). (B) Simultaneously, ACTH induces depolarization of the cell membrane inducing Ca²⁺ influx (Module 3). PKA also activates Ca²⁺ influx through L-type channels. The subsequent increase in intracellular calcium (Cai) activates Ca²⁺-CaMK and steroidogenesis (Module 6). (C) Activated MC2R also interact with ECM and cytoskeleton-associated proteins (Module 4), modulating the phosphorylation and activation of a number of proteins that are involved in functional integrity of the cells. A decrease in paxillin phosphorylation and activation of the phosphotyrosine phosphatase, SHP2, itself activated by PKA-dependent serine phosphorylation is responsible for the rapid effect of ACTH on the rounding-up of adrenocortical cells in culture. SHP2 also induces dephosphorylation of specific substrate(s), including some involved directly or indirectly in steroidogenesis, such as the acyl-CoA synthetase (ACS4), which sequesters AA as arachidonyl-CoA (AA-CoA) (Module 5), hence participating in StAR activation and initiation of steroidogenesis (Module 6). Cytoskeleton-associated proteins and/or PKA are also implicated in the activation of the MAPK signaling, necessary to promote the trophic action of ACTH (Module 5). Clearly identified pathogenic mutations of key proteins are indicated in red. Among these mutations are loss of function of MC2R or MRAPs, activating mutations of the GNAS gene (encoding Gsα subunit), inactivating mutations of genes encoding the regulatory subunit of PKA (Ria) (PRKAR1A), encoding phosphodiesterases (PDE11A and PDE8B) or Aldo-keto- reductases (AKR1B1). Some mutations in voltage-dependent K⁺ channels are directly involved in primary aldosteronism, in particular mutations of the KCNJ5 gene encoding the potassium channel Kir3.4 (also called G-protein-activated inward rectifier potassium channel 4, GIRK4), and of the two genes KCNQ1 and KCNE1, encoding the pore- and regulatory subunits of the slowly activating delayed K⁺ current, I_ks. The resulting sustained Ca²⁺ influx increases activation of CYP11B2 and thus sustained increase in aldosterone secretion. Finally, the temporal integration of these signaling pathways may be coordinated at the levels of signaling microdomains, for example, through A kinase-anchoring proteins, or AKAPs (not illustrated).

• © 2016 Society for Endocrinology